Watch the first 20 seconds of this video.
I am trying to do that exact same thing. The only difference is that I am taking the Mixamo animation, importing it into blender to fix animation direction, then exporting it before I follow the tutorial. However, whenever I select the animation that I want my armature to copy, this happens:
As you can see, it slightly copies the animation but my armature's arms go into a T-pose position. Does anyone know why this might be happening? The tutorial works completely fine if I use the fbx file downloaded straight from the mixamo website, but if I open the fbx in blender and export it as an fbx without changing anything, this happens... Please help!